1. Field of the Invention
Aspects of the present invention relate to devices and systems for measuring the specific absorption rate (SAR) for wireless and other electromagnetic radiation emitting devices. Aspects of the present invention also relate to techniques for measuring SAR or thermal effects generally. One aspect of the present invention relates to an optical technique for measuring SAR for wireless and other electromagnetic radiation emitting devices.
2. Background of the Related Art
The Federal Communications Commission (FCC) sets guidelines for the acceptable levels of electromagnetic radiation emitted by various wireless devices used in close proximity to the human body. Similar standards are applied globally. Devices subject to these standards currently include cellular and mobile telephones, and have the potential to further encompass medical devices as wireless technology expands into this field. Because the FCC requirements and other standards dictate which wireless communication device models can be legally sold, compliance is of paramount importance to both manufacturers and retailers. Yet, the emission profile or SAR profile of a particular device can depend in unexpected and effectively unpredictable ways on parameters that are readily changed from model generation to generation (e.g., the shape of the device, shape of the antenna, exposure of various device components). Predicting resulting SAR profiles from design changes is not easily and accurately achieved computationally. Therefore, an accurate, easily implemented and cost effective measurement technique is essential.
Systems and procedures for testing and evaluating wireless devices, such as cellular telephones, in terms of near-field exposure are known. Most, however, are rather cumbersome, expensive and exhibit considerable limitations, including limitations relating to flexibility, reproducibility and speed and ease of use. Many current devices and methods rely on the physical scanning and mechanical translation of a small electromagnetic field probe through a medium-filled trough, known as a phantom, often having an anthropomorphic shape. See, for example U.S. Pat. No. 7,268,564, the entirety of which is hereby incorporated by reference.
While such devices can effectively provide test results, the robotics and other mechanical translation systems can be expensive and difficult to maintain. Robotics systems and the means to control them with accuracy are inherently expensive. The mechanical probe system described above is also difficult to calibrate and standardize. Testing results on the same subjects have been shown to vary widely in different laboratories and conditions (See “The International Intercomparison of SAR Measurements on Cellular telephones,” Christopher C. Davis, Quirino Balzano, IEEE Transactions on Electromagnetic Compatibility 51, 210-216, 2009, attached as an Appendix and the entirety of which is herby incorporated by reference). Moreover, techniques relying on the mechanical translation of a probe can be relatively slow and not easily applied to large numbers of test samples. In each SAR measurement, probes must be painstakingly rastered throughout a three dimensional, irradiated volume of the phantom liquid, in a way that does not disturb that liquid substantially enough to affect the measurement. This limitation provides an upper bound on the speed with which the technique can ascertain the SAR of a particular device in a specific test media. In fact, a full assessment for a single wireless device can take a skilled operator a day or more to complete.
Further, the necessity of having a fluid-filled phantom that has at least one side open to the air to accommodate the probe and robotic arm creates restraints on the technique's applicability. For example, only tissue-simulating liquid that does not evaporate may be used. Such tissue-simulating liquid almost necessarily includes expensive evaporation-resistive chemical additives that can make dealing with the fluid difficult, cumbersome and, in some cases, hazardous. For example, one commonly used additive to prevent evaporation is Diethylene glycol butyl ether (DGBE), which has been shown to cause birth defects in animals, as well as corneal injuries upon exposure to the eyes. Readily available, cheaper and more benign alternatives for the phantom fluid are necessarily excluded from use in mechanical probe systems with exposed phantom fluid because of the evaporation problem. For example, mixtures of salt water and alcohol can mimic the dielectric properties of the human body at many desired testing frequencies. However, these mixtures cannot be used with open phantoms because the alcohol readily causes them to evaporate. In addition, the nature of the mechanical probe apparatus precludes the use of solid or gelled phantom media.
Current methods involving robotic probes also have physical limitations that can prevent them from being useful for higher frequency tests. Generally speaking, higher frequency signals in the radio frequency range emitted by wireless devices do not penetrate as far into the body (or phantom medium) as lower frequency signals. Then, as wireless devices emit higher and higher frequency radiation, testing will require accurate measurements in smaller and smaller volumes of phantom liquid. This constraint will further require measurements to be made very close to the surface of the phantom. Typical mechanical probes operated by robotics are often too bulky, and their movements too coarse, to provide accurate data in these situations. Even if a suitable robotics operated mechanical probe were to become available, it would likely be prohibitively expensive.
In addition, techniques described above indirectly measure the SAR by measuring the variation of electric fields within the phantom. The energy released into the material of the phantom then needs to be deduced, indirectly, from this electric field measurement. A more direct measurement would be to assess the actual energy released in the phantom via an increase in temperature. From such a measurement, the local energy deposition could be directly calculated, as long as the specific heat of the media in the phantom was known.
Thus, there is a need in the art for a fast SAR assessment apparatus that avoids the use of mechanical and robotic probes and, instead, relies on a less cumbersome, less expensive and more readily available equipment to produce results more quickly. There is also a need in the art for a fast SAR assessment apparatus that allows the use of phantoms that can be completely or nearly completely sealed and, therefore, can accommodate less toxic and more easily handled phantom media. Further, there is a need in the art for a fast SAR assessment apparatus that is adaptable without serious modification to SAR assessment of devices emitting high frequency RF radiation.